Vari-focal system having short total length

Abstract

A vari-focal lens system having short total length comprising, in the order from the object side, a vari-focal subsystem consisting of a first lens unit having a positive refractive power and a second lens unit having a negative refractive power; and an imaging subsystem consisting of a third lens unit having a positive refractive power and always kept stationary, and a fourth lens unit having a positive refractive power and movable for correcting deviation of image point; said third lens unit being composed of four or a smaller number of lens elements including a positive lens element which is arranged on the object side in the third lens unit and has a convex surface on the object side, and a negative lens element, and said imaging subsystem comprising at least one aspherical surface.

We claim: 
     
       1. A vari-focal lens system comprising, in order from the object side, a vari-focal subsystem comprising a first lens unit having a positive refractive power and a second lens unit having a negative refractive power; and   an imaging subsystem comprising a third lens unit having a positive refractive power, and a fourth lens unit having a positive refractive power and being movable during the variation of local length mainly for correcting deviation of the image position;   said third lens unit consisting of two to four lens components including a positive lens component on the object side and having a convex surface on the object side, and a negative lens component having a concave surface on the image side,   said fourth lens unit consisting of two or a smaller number of positive lens components, and   at least one of the lens surfaces in said imaging subsystem being an aspherical surface having refractive power weakened as the portions of said aspherical surface are farther from the optical axis.   
     
     
       2. A vari-focal lens system according to claim 1 adapted to be focused by moving said fourth lens unit along the optical axis. 
     
     
       3. A vari-focal lens system according to claim 1 satisfying the conditions (1), (2) and (3) listed below:   0.55<f.sub.I /{f.sub.T (f.sub.W ·f.sub.T).sup.1/2 }.sup.1/2 <1.35 (1)       0.4<f.sub.III /(f.sub.W ·f.sub.T).sup.1/2 2.5     (2)       -0.5<HH.sub.II-III /(f.sub.W ·f.sub.T).sup.1/2 <0.25 (3)     wherein the reference symbols f W  and f T  represent the focal lengths of the vari-focal lens system as a whole at the wide position and the tele position respectively, the reference symbols f I  and f III  designate the focal lengths of the first lens unit and the third lens unit respectively, and the reference symbol HH II-III  denotes the distance as measured between the principal points of the second lens unit and the third lens unit at the tele position.   
     
     
       4. A vari-focal lens system according to claim 3 wherein said third lens unit comprises a negative lens component arranged on the image side. 
     
     
       5. A vari-focal lens system according to claim 4 wherein said third lens unit and said fourth lens unit satisfy the conditions (4), (5) and (6) mentioned below:   0.3<R.sub.31 /(N.sub.31 -1)(f.sub.W ·f.sub.T).sup.1/2 <2.0 (4)       0.2<R.sub.36 /(N.sub.33 -1)(f.sub.W ·f.sub.T).sup.1/2 <1.6 (5)       0.25<(D.sub.III +D.sub.IV)/(f.sub.W ·f.sub.T).sup.1/2 1.5 (6)     wherein the reference symbols R 31  and R 36  represent the radii of curvature on the object side surface and the image side surface respectively of said third lens unit, the reference symbols D III  and D IV  designate the total thickness of the third lens unit and that of the fourth lens unit respectively, and the reference symbols N 31  and N 33  denote the refractive indices of the lens component arranged on the object side and the lens component arranged on the image side respectively in the third lens unit.   
     
     
       6. A vari-focal lens system according to claim 4 wherein said third lens unit and said fourth lens unit satisfy the conditions (7) through (10) mentioned below:   1.0×10.sup.-3 <D.sub.FT /f.sub.T .sup.2 <7.0×10.sup.-3 ( 7)       -1.8<(R.sub.31 +R.sub.32)/(R.sub.31 -R.sub.32)<-0.1        (8)       0.2<(R.sub.35 +R.sub.36)/(R.sub.35 -R.sub.36)<3.5          (9)       0.6<R.sub.41 /R.sub.36 <2.4                                (10)     wherein the reference symbols R 31  and R 32  represent the radii of curvature on the object side surface and the image side surface respectively of the lens component arranged on the object side in the third lens unit, the reference symbols R 35  and R 36  designate the radii of curvature on the object side surface and the image side surface respectively of the lens component arranged on the image side in the third lens unit, the reference symbol R 41  denotes the radius of curvature on the object side surface of the fourth lens unit, and the reference symbol D FT  represents the distance as measured between the third lens unit and the fourth lens unit at the tele position.   
     
     
       7. A vari-focal lens system according to claim 4 wherein said aspherical surface satisfies the following conditions (11) and (12): ##EQU7## wherein the reference symbol Δx represents the departure of said aspherical surface from the reference sphere thereof, the reference symbol y designates the distance as measured from the optical axis to a portion of interest on said aspherical surface and the reference symbol y 1  denotes a value expressed as ##EQU8## (the reference symbol F represents the F number of the vari-focal lens system at the wide position). 
     
     
       8. A vari-focal lens system according to claim 1 wherein said third lens unit consist of, in the order from the object side, a positive lens component having a convex surface on the object side, two positive lens components and a negative lens component having a concave surface on the image side, and said fourth lens unit consists of a positive lens component. 
     
     
       9. A vari-focal lens system according to claim 1 wherein said third lens unit consists of, in the order from the object side, a positive lens component having a convex surface on the object side, a positive lens component and a negative lens component having a concave surface on the image side, and said fourth lens unit consists of a positive lens component having an aspherical surface. 
     
     
       10. A vari-focal lens system according to claim 1 wherein said third lens unit consists of, in the order from the object side, a positive lens component having a convex object side surface designed as an aspherical surface having curvature lowered as the portions of said aspherical surface are farther from the optical axis, a positive lens component and a negative lens component having a concave surface on the image side, and said fourth lens unit consists of a positive lens component. 
     
     
       11. A vari-focal lens system according to claim 1 wherein said third lens unit consists of, in the order from the object side, a positive lens component having a convex object side surface designed as an aspherical surface having curvature lowered as the portions of said aspherical surface are farther from the optical axis and a negative lens component having a concave surface on the image side, and said fourth lens unit consists of a positive lens component. 
     
     
       12. A vari-focal lens system according to claim 1 wherein said third lens unit consists of, in the order from the object side, a positive lens component having a convex surface on the object side, a negative lens component having a concave surface on the image side and a positive lens component, and said fourth lens unit consists of a positive lens component having an aspherical surface. 
     
     
       13. A vari-focal lens system according to claim 1 wherein said third lens unit consists of, in the order from the object side, a positive lens component having a convex surface on the object side, a positive lens component and a negative lens component having a concave surface on the image side, and said fourth lens unit consists of two positive lens components, and said imaging subsystem comprises an aspherical surface. 
     
     
       14. A vari-focal lens system according to claim 1 satisfying the following condition (1'): ##EQU9## wherein the reference symbols f W  and f T  represent the focal lengths of the vari-focal lens system as a whole at the wide position and the tele position respectively, and the reference symbol f II  designates the focal length of said second lens unit. 
     
     
       15. A vari-focal lens system according to claim 1 satisfying the following condition (2'): ##EQU10## wherein the reference symbols f W  and f T  represent the focal lengths of the vari-focal lens system as a whole at the wide position and tele position respectively, and the reference symbol f IV  deisgnates the focal length of said fourth lens unit. 
     
     
       16. A vari-focal lens system according to claim 1 satisfying the following condition (3'): ##EQU11## wherein the reference symbols f W  and f T  represent the focal lengths of the vari-focal lens system as a whole at the wide position and the tele position respectively, and the reference symbol HH II-III  designates the distance as measured between the principal points of the second lens unit and the third lens unit at the tele position. 
     
     
       17. A vari-focal lens system comprising, in the order from the object side, a first lens unit having a positive refractive power and kept stationary during variation of focal length;   a second lens unit having a negative refractive power, movable during the variation of focal length and having a vari-focal function;   a third lens unit having a positive refractive power and kept stationary during the variation of focal length; and   a fourth lens unit having a positive refractive power, movable during the variation of focal length and having a function mainly to correct deviation of image position;   said third lens unit consisting of two to four lens components including a positive lens component arranged on the object side and having a convex surface on the object side, and a negative lens component,   said fourth lens unit consisting of two or a smaller number of positive lens components, and   said fourth lens unit comprising at least one aspherical surface having refractive power weakened as the portions of said aspherical surface are farther from the optical axis.   
     
     
       18. A vari-focal lens system according to claim 17 wherein said fourth lens unit satisfies the following condition:   -1.8<(R.sub.1 +R.sub.2)/(R.sub.1 -R.sub.2)<-0.1     wherein the reference symbols R 1  and R 2  represent the radii of curvature on the object side surface and the image side surface respectively of the lens component having said aspherical surface in the fourth lens unit.   
     
     
       19. A vari-focal lens system according to claim 17 wherein said negative lens component is arranged on the image side of said positive lens component. 
     
     
       20. A vari-focal lens system according to claim 17 wherein the two lens component arranged on the image side in said third lens unit are a positive lens component having a strongly convex surface on the object side and a negative lens component having a strongly concave surface on the image side.